1. The Field of the Invention
The present invention is related to optical devices for display imaging systems. More particularly, the present invention relates to optical interference filters used to achieve color separation within color projection display imaging systems utilizing liquid crystal light valves for image modulation. The optical interference filters provide improved display brightness and contrast.
2. The Relevant Technology
Projection displays comprise a light source, a polarizing element, a color separation element, reflective imagers such as reflective liquid crystal valves with appropriate drive circuitry, and image projection optics. The polarizing element receives light from the light source and directs one polarization state to the color splitter and transmits the other polarization state. The color splitter utilizes optical interference filters to separate the broad spectrum of light it receives from the light source into three primary colors, directing the separate colors towards the reflective liquid crystal light valves. When the individual pixels of the liquid crystal light valves are activated to an "on" state, they reverse the polarization of incident light upon reflection. The light reflected from the imager is recombined within the color splitter and re-directed to the polarizing element. The polarizing element then acts as an analyzer and transmits light which has undergone a reversal of polarization state at the liquid crystal light valve into the image projection optics, wherein an image is projected that is observable by a viewer. Since light which is not reversed in polarization by the liquid crystal light valve is not transmitted to the image projection optics, the final image is formed from the selected pixels of the three imagers and consists of the three primary colors.
The optical interference filters used in the color splitter are dichroic mirrors designed to reflect and transmit discrete wavelength ranges. Spectral characteristics of these coatings control the display color purity, brightness and contrast. More specifically, the spectral characteristics must be considered over an angular range of incident light controlled by the f# representative of the optical system for S and P planes of polarized light.
The spectral characteristics of optical interference filters or coatings must be considered over an angular range of incident light as such coatings vary in performance with the angle of incidence. To appreciate the potential for variation in spectral characteristics, consider the angular range of incident light which, for example, has a central ray at 30.degree. and has a 10.degree. cone. The resulting angle of incidence will range from 20.degree. to 40.degree.. The difficulties presented in separating light into different colors having the desired characteristics when the light is incident on the dichroic coating with such a broad angular range of incident light become more exaggerated at high angles of incidence. High angles of incidence are generally angles of about 20.degree. or greater.
One of the spectral characteristics impacted by the angular range of incident light which is particularly exaggerated at high angles of incidence is known as angle shift. In an edge or band pass filter, a transition region between the pass band, or high transmission region, and stop band, high reflection region, will shift towards shorter wavelengths with increasing angle of incidence.
Additionally, the S polarization state will shift a greater amount than the P polarization state with angle of incidence. The difference in shift with angle of incidence is known as "polarization splitting" and will result in decreased light transmission. The decreased light transmission occurs because of the double pass nature of the color projection display imaging system, wherein light representing the pixels in the "on" state must be transmitted in one polarization state and reflected in another to be transmitted through the polarizing element into the image projection optics. The average transmitted intensity is proportional to the product of the S and P reflectivities at a given wavelength. Thus, polarization splitting reduces the wavelength range of light available from the light source to form the image.
Several design methods can be used to achieve non-polarizing dichroic coatings; however, the non-polarizing characteristic is limited to a fixed angle of incidence. In practice the system must be optimized to deal with the range of divergent angles. This requires a compromise between two characteristic properties of interference coatings at increasing angles of incidence: 1) the shift of the spectral curve shape towards shorter wavelengths; and 2) increased separation between the S and P polarization states.
This problem can be solved to some extent by using non-polarizing filter designs which align the transition region of both polarization states at one edge, or side, of the filter's pass band. Such design methods for two coating materials are taught by Alfred Thelen in his text entitled Design of Optical Interference Cotig (1989) which is incorporated by reference, and also in U.S. Pat. No. 4,373,782, issued to Thelen which is also hereby incorporated by reference. The optical thin film filter disclosed in the Thelen patent reduces the polarization splitting through the use of carefully designed stacks of low and high refractive index materials which were respectively, n.sub.L =1.45 and n.sub.H =2.28. The Thelen patent, however, is designed for use at only one preselected angle and is therefore most suited to applications such as wavelength division multiplexing and demultiplexing in a fiber optic communications system. Additionally, the method disclosed by Thelen for designing non-polarizing edge filters, requires complex multilayer designs and more than two coating materials.
Other publications teach how to create non-polarizing edge filters using three coating materials. However, this technique is only effective for a given angle of incidence which requires that the light be completely collimated within the color separation and image formation optical path. Such a system is generally not desirable because it further reduces the total light available from the light source, adds expensive components and increases the size of the optical system.
Accordingly, there is a need for improved optical coatings, devices, systems and methods which overcome or avoid the above difficulties.